Role of Silicon as Structural Stabilizer and Redox-Active Element in Multicomponent (Sn, Sb, Fe, Si) Alloy Electrodes for Na-Ion Batteries

Abstract

Metal alloys are attractive electrode materials for sodium-ion batteries (SIBs) thanks to their high theoretical capacities. Because of its good availability and promising use in Li-ion batteries, silicon is also interesting for SIBs. However, so far its use has not been very successful because formation of the NaSi phase (954 mAh g⁻¹) is kinetically unfavorable. Here, the role of Si in multicomponent alloys composed of redox-active metals tin (Sn), antimony (Sb), and redox-inactive iron (Fe) is investigated. The composites are prepared by high-energy ball milling. The resulting particles and agglomerates are typically in the lower μm range (0.5–10 μm) and contain, depending on the composition, the different metals and the intermetallic phases SnSb, FeSn, FeSn₂, FeSi and FeSi₂. It is found that Si remains electrochemically inactive irrespective of its chemical state, but still provides some benefits. Similar to Fe, Si acts as a structural stabilizer for composite electrodes. Excellent rate capability is demonstrated by the Sn:Si:Fe alloy, while the highest capacity and long-term stability are found for the Sn:Sb:Si (2:2:1) alloy. The stabilizing effect of Si (and Fe) is observed through operando electrochemical dilatometry, which shows a much smaller degree of electrode breathing compared to the Si/Fe-free electrode.